Equine infectious anemia virus (EIAV) is the only natural lentiviral infection that causes fulminant clinical disease followed by recovery and a lifelong persistent but subclinical infection. Subclinically infected horses are weakly to undetectably viremic, remain clinically normal indefinitely, but still contain infectious virus. As such, these horses are analogous to human immunodeficiency virus (HIV) patients on highly active antiretroviral therapy (HAART), and are a unique and potentially very useful model of a well-controlled lentiviral infection. This proposal is designed to investigate the sites and begin to explore the mechanisms of EIAV persistence. The predominant host cell during acute infection is the tissue macrophage. Preliminary in situ studies have shown that during subclinical infection tissue macrophages remain infected. However, in vitro and in vivo data from our laboratories suggests that vascular endothelial cells are involved in persistence. These data include in vivo and in vitro infections, noncytopathic infections in vitro, absence of cellular activation in vitro, unique LTR enhancer sequences associated with subclinical infections, and evidence that these LTR sequences would probably function poorly in macrophages. We propose the hypothesis that endothelial cells are the sites of persistence during subclinical EIAV infections. This will be tested as follows: (1) identify the cellular reservoirs and sites of replication of EIAV during subclinical infection. In situ PCR, in situ hybridization, and immunochemistry will be used to characterize the phenotype(s), activation status, and presence of apoptosis in cells that are infected and/or replicating EIAV. (2) examine viral LTR sequences and host cell transcription factors required for EIAV replication in endothelial cells, and the evolution of LTR sequences during changes in host cell tropism in vitro, as well as changes in clinical disease status. Completion of these experiments will further define the EIAV model of lentiviral persistence. In particular, they will define the infected cell type(s) required for persistence, mechanisms of immunologic evasion and transcriptional restriction, and the relationship between different cellular reservoirs in subclinical infection. These studies will provide insight into HIV persistence in HAART patients, and potentially provide a model that can be used to evaluate strategies for actual elimination of lentiviral infections.